LM2876 datasheet(17/22 Pages) NSC

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Application Information (Continued)

01177515

The load current I

L will be much larger than input bias current

I, thus V1 will follow the output voltage directly, i.e. in phase.

Therefore the voltage appearing at the non-inverting input is

effectively positive feedback and the circuit may oscillate. If

there were only one device to worry about then the values of

1 and R2 would probably be small enough to be ignored;

however, several devices normally comprise a total system.

Any ground return of a separate device, whose output is in

phase, can feedback in a similar manner and cause insta-

bilities. Out of phase ground loops also are troublesome,

causing unexpected gain and phase errors.

The solution to most ground loop problems is to always use

a single-point ground system, although this is sometimes

impractical. The third figure below is an example of a single-

point ground system.

The single-point ground concept should be applied rigor-

ously to all components and all circuits when possible. Vio-

lations of single-point grounding are most common among

printed circuit board designs, since the circuit is surrounded

by large ground areas which invite the temptation to run a

device to the closest ground spot. As a final rule, make all

ground returns low resistance and low inductance by using

large wire and wide traces.

Occasionally, current in the output leads (which function as

antennas) can be coupled through the air to the amplifier

input, resulting in high-frequency oscillation. This normally

happens when the source impedance is high or the input

leads are long. The problem can be eliminated by placing a

small capacitor, C

C, (on the order of 50 pF to 500 pF) across

the LM2876 input terminals. Refer to the External Compo-

nents Description section relating to component interaction

with C

REACTIVE LOADING

It is hard for most power amplifiers to drive highly capacitive

loads very effectively and normally results in oscillations or

ringing on the square wave response. If the output of the

LM2876 is connected directly to a capacitor with no series

resistance, the square wave response will exhibit ringing if

the capacitance is greater than about 0.2 µF. If highly ca-

pacitive loads are expected due to long speaker cables, a

method commonly employed to protect amplifiers from low

impedances at high frequencies is to couple to the load

through a 10

Ω resistor in parallel with a 0.7 µH inductor. The

inductor-resistor combination as shown in the Typical Ap-

plication Circuit isolates the feedback amplifier from the

load by providing high output impedance at high frequencies

thus allowing the 10

Ω resistor to decouple the capacitive

load and reduce the Q of the series resonant circuit. The LR

combination also provides low output impedance at low

frequencies thus shorting out the 10

Ω resistor and allowing

the amplifier to drive the series RC load (large capacitive

load due to long speaker cables) directly.

GENERALIZED AUDIO POWER AMPLIFIER DESIGN

The system designer usually knows some of the following

parameters when starting an audio amplifier design:

Desired Power Output

Input Level

Input Impedance

Load Impedance

Maximum Supply Voltage

Bandwidth

The power output and load impedance determine the power

supply requirements, however, depending upon the applica-

tion some system designers may be limited to certain maxi-

mum supply voltages. If the designer does have a power

supply limitation, he should choose a practical load imped-

ance which would allow the amplifier to provide the desired

output power, keeping in mind the current limiting capabili-

ties of the device. In any case, the output signal swing and

current are found from (where P

O is the average output

power):

(5)

(6)

To determine the maximum supply voltage the following

parameters must be considered. Add the dropout voltage

(4V for LM2876) to the peak output swing, V

opeak, to get the

supply rail value (i.e. ± (V

opeak + Vod) at a current of Iopeak).

The regulation of the supply determines the unloaded volt-

age, usually about 15% higher. Supply voltage will also rise

10% during high line conditions. Therefore, the maximum

supply voltage is obtained from the following equation:

Max. supplies

≈ ± (V

opeak + Vod)(1 + regulation)(1.1)

(7)

The input sensitivity and the output power specs determine

the minimum required gain as depicted below:

(8)

Normally the gain is set between 20 and 200; for a 40W, 8

Ω

audio amplifier this results in a sensitivity of 894 mV and

89 mV, respectively. Although higher gain amplifiers provide

greater output power and dynamic headroom capabilities,

there are certain shortcomings that go along with the so

called “gain.” The input referred noise floor is increased and

hence the SNR is worse. With the increase in gain, there is

also a reduction of the power bandwidth which results in a

decrease in feedback thus not allowing the amplifier to re-

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